Yuchen Guo

Yuchen Guo

A PhD candidate at Department of Physics, Tsinghua University

Talks and presentations

A New Framework for Quantum Phases in Open Systems: Steady State of Imaginary-Time Lindbladian Evolution

October 17, 2024

Talk, Koushare, Online

In this talk, I plan to introduce our two recent works (arXiv 2403.16978 and arXiv 2408.03239) on topological phases in open quantum systems. I will begin with reviewing the classification of quantum phases in closed systems and properties of corresponding phase transitions. Next, I will discuss our tensor network construction for a specific class of topological states, namely average symmetry protected topological (ASPT) phases, defined in open systems, especially with a nontrivial extension of strong and weak symmetry. I will then introduce a new framework to systematically study the phase transitions between different open system quantum phases with the imaginary-time Lindbladian evolution. To illustrate the effectiveness of this framework, we apply it to investigate the phase diagram for open systems with Z_2^σ×Z_2^τ symmetry, including cases with nontrivial ASPT order or spontaneous symmetry breaking (SSB) order. I will finally discuss several universal properties at quantum criticality, such as nonanalytic behaviors of steady-state observables, divergence of correlation lengths, and closing of the imaginary-Liouville gap. These results advance our understanding of quantum phase transitions in open quantum systems.

Non-Hermitian Parent Hamiltonian and Composite Quantum Phases

April 27, 2023

Talk, Koushare, Online

In this talk, I plan to introduce our two recent works (arXiv: 2301.12448, arXiv: 2304.04588) on non-Hermitian interacting spin systems. I will begin with introducing the basic concept of tensor networks, and the conventional parent Hamiltonian method for Hermitian systems. Next, I will discuss our proposed non-Hermitian parent Hamiltonian (nH-PH), where one can start from two different matrix product states (MPS) and construct a local Hamiltonian such that these states are zero-energy modes. I will then introduce a class of new topological phases in non-Hermitian interacting systems without Hermitian counterparts, denoted as composite quantum phases. With the nH-PH approach, we construct a spin-1 model to realize this type of exotic phases. I will finally show the phase diagram of our model and demonstrate that such novel phases can exist extensively in non-Hermitian systems.

Quantum noise effects and quantum error mitigation

November 02, 2022

Talk, Institute for Interdisciplinary Information Sciences, Tsinghua University, MMW s327

In this talk, I plan to introduce our two recent works (arXiv: 2201.00752, arXiv: 2207.01403). I will begin with briefly introducing the basic concepts of quantum error mitigation (QEM) in the era of noisy intermediate-scale quantum (NISQ) devices and a typical QEM approach, namely the quasi-probability decomposition method. I will then make comments on this method from two aspects, i.e. its powerlessness on correlated noise and the physical meaning of its sampling cost, respectively. In the remaining time, I will focus on these two issues. In our first work (arXiv: 2201.00752), we use matrix product operators (MPO) to represent the noise channel to characterize and mitigate correlated noise in quantum circuits. As for the second work (arXiv:2207.01403), we find that the physical implementability, which is the sampling cost of implementing the noise inverse, is a good characterization of the decoherence effects.